Quantitative assessment of fatigue damage accumulation in wavy slip metals from acoustic harmonic generation

A comprehensive, analytical treatment is presented of the microelastic-plastic nonlinearities resulting from the interaction of a stress perturbation with dislocation substructures and cracks that evolve during cyclic fatigue of wavy slip metals. The interaction is quantified by a material nonlinearity parameter beta extracted from acoustic harmonic generation measurements. The contribution to beta from the substructures is obtained from the Cantrell model. The contribution to beta from cracks is obtained by applying the Paris law to the Nazarov-Sutin crack nonlinearity equation. The nonlinearity parameter resulting from the two contributions is predicted to increase monotonically by hundreds of percent during fatigue from the virgin state to fracture. The increase in beta during the first 80-90% fatigue life is dominated by the evolution of dislocation substructures, while the last 10-20% is dominated by crack growth. Application of the model to aluminium alloy 2024-T4 in stress-controlled loading at 276MPa yields excellent agreement between theory and experiment.